An image forming apparatus comprises, for example, a power supply device for supplying power to an electrical load, such as a transfer roller. The power supply device defines a current value of power to be supplied to an electrical load as a control target value, and conducts a constant current control for keeping this control target value at a target value. For this purpose, a detection circuit, which detects the above-mentioned control target value and outputs a voltage signal according thereto, is provided in the power supply device, and a control circuit of the power supply device is configured to receive a voltage value of the above-mentioned voltage signal as a feedback value.
A control circuit in a power supply device is generally configured on the basis of receiving a positive voltage value as a feedback value. However, the polarity of the feedback value is inverted to negative at the time when, for example, the power supply device is turned off. If so, the above-mentioned control circuit receives a voltage outside of a prescribed range (for example, an assured range), and thus may possibly be badly affected.